import java.util.*;
public class Main{
public static void main(String[]args)
{
Scanner sc=new Scanner(System.in);
if(!sc.hasNext())
{
System.out.println("Invalid input");
return;
}
String s=sc.next();
s=s.toLowerCase();
for(char c:s.tocharArray())
{
char ch=s.charAt(c);
if(!Character.isLetter(ch))
{
System.out.println("Invalid input");
return;
}
}
if(s.length()==0||s.length()%2!=0)
{
System.out.println("Invalid input");
return;
}
int start=0;
int close=0;

int count=0;
if(s.length()%2==0)
{
count=s.length()/2;
System.out.println(count);
}
for(int i=0;i<s.length();i++)
{
char c=s.charAt(i);
if(c=='a')
{
start++;
}
else if(c=='b')
{
close++;
if(close>start)
{
imbalance++;
}
}
else{
System.out.println("Invalid input");
return;
}
}



}
}